Influence of the Amsterdam/St. Paul hot spot along the Southeast Indian Ridge between 77° and 88°E: Correlations of Sr, Nd, Pb, and He isotopic variations with ridge segmentation

• Main Authors: Nicolaysen, K. P., Frey, F. A., Mahoney, J. J., Johnson, K. T. M., Graham, David W.
• Other Authors: College of Oceanic and Atmospheric Sciences
• Format: Article in Journal/Newspaper
• Language: English; unknown
• Published: American Geophysical Union
• Subjects: Paul Islands
• Online Access: https://ir.library.oregonstate.edu/concern/articles/cc08hm597

The submarine Amsterdam-St. Paul (ASP) Plateau, bisected by the Southeast Indian Ridge (SEIR), is a bathymetric high rising ~2 km above the surrounding seafloor that includes the islands of Amsterdam and St. Paul; this excess volcanism is attributed to a mantle hot spot. We obtained new Sr, Nd, and Pb (n = 37) and He isotopic (n = 10) ratios for basalt glasses from 11 SEIR segments on and adjacent to the plateau and from three seamounts on the plateau. The results show systematic spatial variations in these isotopic ratios that correlate with physical segmentation of the ridge. Specifically, lavas from the four ridge segments on the ASP Plateau have higher ²⁰⁸Pb/²⁰⁴Pb at a given ²⁰⁶Pb/²⁰⁴Pb than SEIR basalts distant from ASP Plateau. Surprisingly, lavas from the ridge segment 100 km north of the ASP Plateau are distinguished by the most radiogenic ²⁰⁶Pb/²⁰⁴Pb (up to 19.4) and highest ³He/⁴He ratios (up to 14.1 RA). These are characteristics of lavas erupted at Amsterdamand St Paul Islands. The isotopic data for SEIR basalts erupted on or adjacent to the ASP Plateau provide equivocal evidence for a mantle component derived from the distant Kerguelen hot spot. Overall, the Pb-Nd-Sr-He isotope variations within this data set are explained well by three mantle endmembers: (1) depleted mantle having relatively low ²⁰⁶Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd, which has been variably mixed with (2) material having relatively high ²⁰⁸Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd, a signature commonly ascribed to detached or eroded metasomatized continental lithosphere, and (3) hot spot–related mantle having elevated ³He/⁴He and ²⁰⁶Pb/²⁰⁴Pb but intermediate ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, similar to the common or C material observed in hot spots globally. These results suggest either that the ASP hot spot is isotopically heterogeneous or that the shallow mantle or lithosphere beneath the ASP Plateau contains more continentally derived material than the SEIR mantle ≥500 km away. Perhaps, like the 39°–41°E section of the ...